1. Field of the Invention
The invention relates to a layered electret retaining a high surface potential and a method of making the same from synthetic high molecular weight substances.
2. Description of the Prior Art
Electrets made out of a plastic sheet of film consisting of a low molecular weight organic compound, such as carnauba wax or naphthalene, a non-crystalline high molecular weight compound, such as poly(methyl methacrylate) or polystyrene, a crystalline high molecular weight compound, such as poly(ethylene terephthalate), polycarbonates, poly(fluorinated ethylenes) or polypropylene, or a copolymer or mixtures thereof, made by holding the sheet or film at a proper high temperature for a long period of time and then cooling it to room temperature while imposing thereto a high voltage of direct current through the heating and cooling stages, retain their polarized state for a long period of time, so that a variety of uses therefor, e.g., in speakers, microphones and like electricity-sound converters, or in other electronic appliances, have been taken into consideration. Especially, suitable materials used in forming electrets of relatively long life are polar high molecular weight compounds; such as, e.g., poly(methyl methacrylate), poly(ethylene terephthalate), polycarbonates or polar fluoro resins. However, electrets formed of such materials do not always retain a high enough surface potential for application in an electricity-sound converter or other electronic applicance.
Although it is necessary in obtaining an electret having a high surface potential to use an electret material of a sufficient thickness, the use of a thick electret material is unpractical, since it requires an electric field of a high voltage for polarization. Therefore, it is common in methods of producing electrets having a high surface potential to convert a relatively thin plastic sheet material in a conventional manner to an electret and then building up the electret in layers. In many cases, the electret is put in use under conditions which are more adverse than the ordinary storage conditions, e.g., under high temperature conditions compared to room temperature, and, under such injurious conditions, even electrets formed of the above-mentioned plastic material, which have hitherto been considered to be relatively stable, cannot always retain their function for a long period of time.
Accordingly, it is the primary object of the present invention to provide an electret which obviates the aforementioned disadvantages.
Other objects and advantages will become apparent from the ensuing description.